This is kind of a difficult question. Thai proper has these basic script (and likely I am forgetting one or two), in order of time1. Ramkhamhaeng script(still debateable on authenticity)2. Sukhothai script3. King Lu(?) Thai script4. King Narai reform script4a . Compressed Thai script4b . Modern Thai script5. King Monkut reform script (didn't catch on)6. And there was another proposed reform that didn't catch on either..will have to look it up tonight.

And somewhere in this list, the Khom script should go. That's Thai written with the Khmer (Aksar-Mul type) script, and a few extra letters and marks for tones.

Tai languages (Lao, Dham, Tai-Deng, Tai-Dam, Lanna, Khuen, Lue, Shan, Tai-Neua, Tai-Lai-Chau, Aiton, Ahom, Khamti, White Tai, SonLa Tai, Pai-Yi, Tai Na, Sipsongpanna, etc) all have their own apparently-Mon derived script....with those in the Yunnan region being kind of diamond-shaped due to their being written with a brush on paper/bamboo rather than on a palm-leaf with a stylus....suffice it to say there are A LOT of different Thai/Tai scripts.

OK, now I found the book on Thai scripts.....some corrections......should have been King Li Thai script, not Lu Thai. Also, the other proposed script that I couldn't remember is the King Rama VI script around 1917....didn't catch on.

The most excellent book "The Thai writing system" by Nanthana Danwiwat (mostly viewable on Google Books) is where you should start. It will probably drive you crazy because some essential pages seem to be missing. For the actual book, I know its available at some big university libraries (UCLA, UCBerkeley, Yale, Harvard).

Keep in mind...that this is only for Thai Proper.....not the other Tai languages and writing.

Very interesting, the link to the Ram Khamhaeng era script. The vowel glyphs table is the obvious clincher. It's fascinating to see how the style of the modern Thai script is such a recent innovation, and how similar the older script's overall ductus is to the Indonesian Kawi although the letterforms have already changed so much in the two scripts that very few are recognisably the same! This script and older Kawi both show the curvy ductus of scripts written on palm leaves.